Environmental monitoring and control systems play a vital role in many industrial applications to ensure proper production and quality. However, these systems traditionally have required manually intensive processes and expensive inflexible equipment.
For example, modern temperature control systems rely on Resistance Temperature Devices (RTDs) for sensing temperature. RTDs are wire wound and thin film devices that work on the physical principle of the temperature coefficient of electrical resistance of metals (i.e. resistance change with temperature). Measuring temperature using RTD technology requires the presence of specialized and highly sensitive electronic circuitry that is capable of accurately measuring small changes in electrical resistance. Typically, RTD interface circuitry produces an output voltage proportional to the RTD resistance, which is either used directly in a voltage comparator system or it is scaled, digitized and then read by a microprocessor where it is converted to temperature via software, based on the published RTD temperature versus resistance characteristics.
The use of RTDs in industrial temperature control applications presents the following problems or issues: (1) RTD interface circuitry must be calibrated for the attached RTD and interconnection components; (2) RTD temperature sensing is error prone in harsh environments; and (3) multiple sensor applications are impractical with RTDs due to wiring constraints and required circuit real estate.
In industrial applications, RTDs are usually located a great distance from the interface circuitry used to read their resistance. The long wires used to make these connections introduce electrical resistance in series with the RTD, which causes a constant temperature offset error that must be adjusted for in the RTD interface circuitry. This lead-wire resistance compensation takes the form of a variable resistor in the RTD interface circuitry and is referred to as offset calibration. The act of calibrating RTD interface circuitry to the particular electrical characteristics of an RTD device and its electrical connection is a labor-intensive process.
RTD interface circuitry must also provide an electrical adjustment that tailors the circuit for the specific “type” of RTD being used. There are many RTD devices, from many different manufactures, each with its unique electrical characteristics (temperature versus resistance). RTD interface circuitry must provide some form of calibration to allow it to accurately read different RTD products (types) or else be specifically designed for one and only one type of device. This calibration facility may be the same adjustment described above, or it may be yet an entirely separate adjustment of its own.
In harsh industrial environments (e.g., high moisture and/or caustic chemicals), the electrical contacts used to connect RTDs to their interface circuitry can become severely corroded, causing significant increases in the electrical resistance, which is in series with the RTD. This increase in series resistance translates directly to temperature measurement errors or offsets. Because each RTD sensor requires its own set of electrical wiring and interface circuitry, multiple sensor probes beyond two sensors are impractical. FIGS. 10–12 illustrate this observation in which the various circuits used to read RTDs are described.
FIG. 10 is diagram of a conventional resistance temperature device (RTD) employing a 2-wire circuit 1000. A Wheatstone bridge is the most common approach for measuring an RTD. As RT increases or decreases with temperature, Vout also increases or decreases. An operational amplifier (op-amp) is used to observe Vout. Lead wire resistance, L1 and L2 directly adds to the RTD leg of the bridge.
FIG. 11 is diagram of a conventional resistance temperature device (RTD) employing a 3-wire circuit 1100. In this approach, L1 and L3 carry the bridge current. When the bridge is in balance, no current flows through L2, thus no L2 lead resistance is observed. The bridge becomes unbalanced as RT changes. An op-amp is used to observe Vout and prevent current flow in L2. The effects of L1 and L3 cancel when L1 equals L3 since they are in separate arms of the bridge.
FIG. 12 is diagram of a conventional resistance temperature device (RTD) employing a 4-wire circuit 1200. The 4-wire circuit 1200 uses a constant current source to cancel lead wire effects even when L1 does not equal L4. The op-amp is used to observe Vout and to prevent current flow in L2 and L3.
Therefore, there is a need for a temperature sensing system that can operate effectively in harsh environments. There is also a need to minimize temperature sensing errors. There is also a need for a temperature sensing system that avoids the time consuming process of calibration. There is a further need for a temperature sensing system that is adaptable to sophisticated and robust monitoring.